Many UPS's have a USB connection to the mac and management software. You can set it up so that when the UPS looses power it sends a message to the machine to shut down.

You do not need an UPS for that. The way you described it, you would end up with some 50% efficiency or even less.

A simple microcontroller with one input line and RS232 port will do the job. They can be easily programmed in C and the program should not take more than 20 lines. Just watch the input pin and send a word/sequence of bytes over the RS232 when the pin goes high (or low). On the Mac side another simple program would listen on the serial port for a pre-defined word and perform "shutdown now" when it receives it. I used to do this back in 2002 in the first version of my install and it worked without any problems.

You do not need an UPS for that. The way you described it, you would end up with some 50% efficiency or even less.

A simple microcontroller with one input line and RS232 port will do the job. They can be easily programmed in C and the program should not take more than 20 lines. Just watch the input pin and send a word/sequence of bytes over the RS232 when the pin goes high (or low). On the Mac side another simple program would listen on the serial port for a pre-defined word and perform "shutdown now" when it receives it. I used to do this back in 2002 in the first version of my install and it worked without any problems.

I switched mine to use the sleep mode which is initiated by the Carnetix power supply for the Mac Mini. It works great! I don't know why I ever wanted it to shut down. It's awesome to have music playing right where I left off and no delay. It does look like some one is finding a solution to powering it off in another thread "shuting down the Mac Mini" using Seepwatcher.

Yes, I still use the stock radio for sound. It has an auxillary input for the sound to come from the Mac Mini. I also have the steering wheel controls for the stock radio that allow me to adjust the volume. I down loaded the USB drivers for the touch screen from the internet. They were compatible with OSX